This invention relates to liquid cooled dynamoelectric machines, such as turbogenerators, the unit capacity of which is extremely large, and more particularly to a construction of rotors of such electric machines which is adapted to circulate liquid coolant.
Heretofore, a dynamoelectric machine of a large capacity has been ordinarily cooled with a gaseous medium, such as hydrogen gas. In such cases, the gaseous medium is forcibly circulated by a fan secured to the rotor through ducts and windings of the stator and rotor for passing through a heat exchanger which is provided within the stator for transmitting the heat of the gaseous medium to outside air.
However, as a result of the constant increase in recent years of the unit capacity of the electric machine, there is a tendency of changing the cooling medium from the gaseous coolant to liquid coolant such as water or oil, the cooling capability of which is much higher than that of the gaseous coolant. Since water is used in most cases, a term "cooling water" is hereinafter used instead of the liquid coolant.
In cases where a liquid coolant (or cooling water) is used in the electric machine of a large capacity, each conductor for the stator windings and rotor windings is formed into a hollow construction having an internal bore through which cooling water is circulated.
Although it is comparatively easy to supply and circulate cooling water through the stator windings of the above described construction, difficulties arise in supplying and exhausting cooling water into and out of the rotor which is rotated at a high speed. It is preferred that cooling water is supplied into and exhausted out of the rotor at positions nearby the rotating axis of the rotor in order to avoid adverse effect of the centrifugal force.
Conventionally, a cooling water supplying and exhausting device has been provided on one longitudinal side of the rotor. The device comprises a cooling water supply tube and a cooling water exhaust tube both extending through a bore of the rotor shaft. Cooling water supplied through the cooling water supply tube is then passed through radial conduits or radial passages into a cooling water distributing chamber provided on the surface of the rotor, and then is distributed throughout the internal bore of the hollow conductors constituting the rotor windings (or field windings). Cooling water circulated through the rotor windings is collected in a chamber similar to the distributing chamber, and returned through radial conduits or radial passages to the cooling water exhaust tube. All of the above described tubes, conduits, and chambers, except the winding conductors, or at least the internal surfaces of these members are made of, or lined with a corrosion resistant material such as stainless steel of austenite structure.
Ordinarily, the cooling water supply tube and the cooling water exhaust tube are arranged concentrically with the supply tube disposed at the center. More specifically, the two tubes are combined into a double wall construction, a space within the internal wall serving as the cooling water supply tube, while a space between the two walls serving as the cooling water exhaust tube.
In the above described conventional arrangement, since the cooling water exhausted from the field windings and therefore having a high temperature is exhausted through the cooling water exhaust tube formed between the two walls, temperature differences are created between the outer wall and the inner wall, and also between the outer wall and the rotating shaft, thus resulting in uneven thermal expansions of these members. The difference in thermal expansions between the outer wall and the rotor shaft is particularly great because of a difference of more than 50% existing between the linear thermal expansion coefficients of these members.
In a conventional construction where the two tube walls are secured to the rotor shaft at the distal and proximal ends of the same walls, excessive stresses tend to be created particularly at the junction points between these tube walls and the radial conduits and also internally of these walls. Each time the electric machine starts and stops, these stresses appear and disappear repeatedly. Should cracks are formed in these parts, serious troubles such as breakage of the rotor shaft and leakage of water tend to occur.
For obviating such difficulties, a construction including flexible bellows at each junction point between the radial conduit and the cooling water supply tube or exhaust tube has been disclosed in U.S. Pat. No. 3,740,595. An alternative construction wherein members tending to be subjected to severe stresses are prestressed in a direction opposite to expected stresses has been disclosed in Japanese Laid-Open Patent Specification No. 102805/1975.
However, the former construction is found to be disadvantageous because of a comparatively short operational life of the bellows, while the latter construction has a difficulty in predicting intensities of such stresses.
Japanese Patent Publication No. 47881/1978 discloses an arrangement wherein mechanical seals and fluid seals are provided for preventing cooling water from leaking out of gaps between the shaft and structural members. However, such a construction is not satisfactory because the sealing members must be replaced occasionally, and the size of the cooling water supplying and exhausting device is thereby increased.